1. Field of the Invention
The present invention relates to a novel hybrid electrolyte. More particularly, the present invention is concerned with a novel hybrid electrolyte comprising a shaped porous polymer structure comprising a polymer matrix and a plurality of cells dispersed in the polymer matrix, the polymer matrix containing a crosslinked polymer segment and having a specific gel content, wherein the shaped porous polymer structure is impregnated and swelled with an electrolytic liquid. The present invention is also concerned with a method for producing the hybrid electrolyte and a method for producing an electrochemical device comprising the hybrid electrolyte.
The hybrid electrolyte of the present invention has a high ionic conductivity, an excellent stability under high temperature conditions and an excellent adherability to an electrode, so that the hybrid electrolyte of the present invention can be advantageously used as an electrolyte for various electrochemical devices, such as primary and secondary batteries (e.g., a lithium battery), a photoelectrochemical device and an electrochemical sensor. Further, by the method of the present invention, the hybrid electrolyte having the above-mentioned excellent properties and an electrochemical device comprising the same can be surely and effectively produced.
2. Prior Art
Recently, for reducing the size and weight of portable equipments, such as pocket telephones and personal computers, there has been a demand for a battery having high energy density. As a battery for meeting such a demand, lithium ion batteries have been developed. This type of battery has a structure in which a porous separator is disposed between the positive and negative electrodes, wherein the porous separator is not swelled with an electrolytic liquid. For preventing a leakage of the electrolytic liquid used for impregnating the separator, the commercially produced battery of this type has a battery structure wholly packaged in a very strong metallic casing having a large thickness.
On the other hand, so-called solid type batteries produced using a solid electrolyte functioning not only as an electrolyte but also as a separator are advantageously free from the danger of leakage of an electrolytic liquid. Therefore, it is expected that not only is a solid electrolyte useful for providing a battery having improved reliability and safety, but is also advantageous in that both the lamination of a solid electrolyte onto electrodes and the packaging of the resultant laminate to form a battery can be easily performed, wherein the thickness and weight of the battery can be reduced. Especially, a polymeric solid electrolyte comprising an ion-conductive polymer has excellent flexibility for processing and, therefore, not only can a laminate structure composed of the polymeric solid electrolyte and electrodes be easily produced, but also the polymeric solid electrolyte is capable of changing its morphology at an interface between the electrolyte and the electrodes in accordance with the volumetric change of the electrodes caused by the occlusion and release of ions by the electrodes, enabling the interface of the polymeric solid electrolyte to intimately fit over the electrodes without suffering delamination from the electrodes.
As such a polymeric solid electrolyte, an alkali metal salt complex of polyethylene oxide was proposed by Wright in British Polymer Journal, vol.7, p.319 (1975). Since then, researches on various skeletal materials for polymeric solid electrolytes have been energetically conducted. Examples of such skeletal materials include polyethers, such as polyethylene oxide and polypropylene oxide, polyphosphazene and polysiloxane. Generally, polymeric solid electrolytes are provided in the form of solid solutions of a solid electrolyte in a polymeric solid, wherein the solid electrolyte is considered to be uniformly dissolved in the polymeric solid, and are known as dry type polymeric solid electrolytes. However, these polymeric solid electrolytes have a problem in that the ionic conductivity of them is extremely low as compared to that of an electrolytic liquid. Therefore, a battery produced using such a polymeric solid electrolyte has problems in that it has a low charge/discharge current density and has a high resistance.
For solving these problems, various attempts to improve the ionic conductivity of a polymeric solid electrolyte have been proposed, wherein the condition of the solid electrolyte is rendered similar to the condition of the electrolyte in the electrolytic liquid. For example, gelled solid electrolytes are known which are obtained by adding a solvent for the electrolyte (which solvent is capable of dissolving an electrolyte to form an electrolytic liquid) as a plasticizer to a polymer matrix so that the solvent and the polymer matrix together form a gel, wherein the solvent is used for increasing the dissociation of the electrolyte and promoting the molecular movement of the polymer, so that the ionic conductivity of the electrolyte can be increased (see, for example, Japanese Patent Application Laid-Open Specification No. 57-143356). As an example of such a gelled solid electrolyte, U.S. Pat. No. 5,296,318 discloses a gelled solid electrolyte obtained by adding an electrolytic liquid to a vinylidene fluoride polymer so that the electrolytic liquid and the polymer together form a gel. Further, U.S. Pat. No. 5,429,891 discloses a gelled solid electrolyte obtained by adding an electrolytic liquid to a crosslinked vinylidene fluoride polymer to thereby swell the crosslinked polymer so that the electrolytic liquid and the crosslinked polymer together form a gel. In general, when a battery comprising such a gelled solid electrolyte (i.e., a so-called hybrid electrolyte) is produced, a hybrid electrolyte comprising a crosslinked polymer swelled with an electrolytic liquid is produced, and then, a battery is assembled using the swelled hybrid electrolyte, electrodes, etc. With respect to the polymer matrix of such a hybrid electrolyte, a crosslinked polymer can be used. On the other hand, a method for producing a battery comprising a hybrid electrolyte layer is also known, wherein the hybrid electrolyte layer is formed by coating electrodes for the battery with a solution obtained by dissolving a non-crosslinked polymer, an electrolyte and a plasticizer in a low boiling point solvent, followed by removing the solvent by evaporation (see U.S. Pat. No. 5,296,318). Each of these materials is electrochemically stable and has a high ionic conductivity, as compared to that of a conventional dry type solid electrolyte. However, the ionic conductivity of each of the above-mentioned hybrid electrolytes is still unsatisfactory, as compared to that of an electrolytic liquid. Further, a non-porous polymer matrix is used for each of the above-mentioned conventional hybrid electrolytes. Hence, the capacities of the batteries comprising such conventional hybrid electrolytes are disadvantageously low.
As a hybrid electrolyte having a high ionic conductivity, a material has been proposed, which comprises a gel phase (comprising a polymer and an electrolytic liquid) and a liquid phase (comprising an electrolytic liquid), wherein the liquid phase is dispersed in the gel phase. For example, Unexamined Japanese Patent Application Laid-Open Specification No 8-250127 describes the use of a vinylidene fluoride porous polymer sheet as a polymer matrix of a solid electrolyte. In this document, a description is made with respect to a method for impregnating a porous polymer sheet with an electrolytic liquid under high temperature conditions, to thereby form a hybrid electrolyte (comprising the porous polymer sheet impregnated and swelled with the electrolytic liquid), which is similar to the hybrid electrolyte of the present invention. Further, Unexamined Japanese Patent Application Laid-Open Specification No. 6-150939 discloses a method for producing a hybrid (solid) electrolyte, in which a porous structure comprising a crosslinked polymer containing polar units is used as a matrix for the hybrid (solid) electrolyte. However, in the method described in these documents, in order to retain an electrolytic liquid in the matrix, a crosslinked porous polymer sheet as the matrix is immersed in an excess amount of an electrolytic liquid under conditions at which a non-crosslinked polymer segment contained in the crosslinked porous polymer sheet can be dissolved in the electrolytic liquid. In the hybrid electrolyte thus obtained, the non-crosslinked polymer segment contained in the crosslinked porous polymer sheet (which segment is capable of imparting the resultant electrolyte with an adherability to electrodes) is dissolved into the electrolytic liquid during the immersion, so that the adherence strength of the resultant electrolyte to electrodes disadvantageously becomes low.
Further, Unexamined Japanese Patent Application Laid-Open Specification No. 8-195220 discloses a method for producing a hybrid electrolyte comprising a porous polymer matrix, which comprises dispersing a non-crosslinked polyacrylonitrile in an electrolytic liquid to thereby obtain a dispersion; coating a stainless steel substrate with the obtained dispersion and heating the dispersion coated on the stainless steel substrate to dissolve the non-crosslinked polyacrylonitrile (which is contained in the dispersion coated on the stainless steel substrate) into the electrolytic liquid, to thereby form a homogeneous solution; cooling the thus formed solution on the stainless steel substrate to thereby form a hybrid electrolyte layer comprising a polymer matrix comprising the non-crosslinked polyacrylonitrile and the electrolytic liquid retained in the polymer matrix; and pricking holes in the hybrid electrolyte layer (by means of a thin stainless needle) in a condition where the hybrid electrolyte layer is immersed in a solution of an electrolyte, so that the polymer matrix of the hybrid electrolyte layer is rendered porous, to thereby obtain a hybrid electrolyte comprising the porous polymer matrix and the electrolytic liquid contained therein. However, in this method, it is required to dissolve a polyacrylonitrile into an electrolytic liquid and, therefore, it is required to use a non-crosslinked polyacrylonitrile. Further, in this method, it is difficult to introduce a crosslinked structure into the polyacrylonitrile constituting the polymer matrix of the hybrid electrolyte obtained by this method Therefore, the non-crosslinked polyacrylonitrile constituting the porous polymer matrix of the hybrid electrolyte is likely to be dissolved into the electrolytic liquid or fused under high temperature conditions, so that there is disadvantageously a danger that the hybrid electrolyte obtained by this method suffers distortion, thereby causing shutting or short-circuiting of the pores of the hybrid electrolyte.
Further, each of the above-mentioned various types of hybrid electrolytes is constructed with a polymer which is already swelled with an electrolytic liquid, so that the mechanical strength of the electrolyte is disadvantageously low and, hence, it is not easy to handle the hybrid electrolyte for laminating the hybrid electrolyte onto electrodes in the assembling of a battery. In particular, it is extremely difficult to produce the above-mentioned hybrid electrolyte in the form of a thin sheet so as to increase the energy density of the hybrid electrolyte. With respect to the method comprising coating an electrolyte with a solution of a polymer and an electrolyte in a solvent, the handling of the electrolyte is easy However, from the viewpoint of safety, this method is not preferred because a low boiling point solvent which is combustible, such as THF, is used.
On the other hand, an attempt to prevent the electrolytic liquid in a solid electrolyte from leakage has been proposed, wherein a liquid ion conductor is filled in the pores of a porous polymer sheet of the solid electrolyte so that it can be retained in the porous polymer sheet by the capillary action. For example, a microporous polymer sheet made of a material having a high mechanical strength, such as a polyolefin, and having a through-hole diameter of 0.1 mm or less is provided, and the pores of the microporous polymer sheet are filled up with an ion transferring medium to thereby form a thin electrolyte sheet (Unexamined Japanese Patent Application Laid-Open Specification No. 1-158051). With respect to the solid electrolyte of this type, the mechanical strength is large; however, a large number of pores in the microporous polymer sheet form complicated labyrinthian passages and, therefore, ions have to pass through the electrolytic liquid phase in such complicated labyrinthian passages, so that the above-mentioned solid electrolyte has a defect in that the ionic conductivity thereof is disadvantageously low.